1. Field of the Invention
The present invention relates to a recording/playback apparatus and method for performing recording/playback on a hologram recording medium to which data is recorded by interference fringes of signal light and reference light. The present invention also relates to a recording apparatus and method for performing recording on a hologram recording medium and a playback apparatus and method for performing playback on a hologram recording medium.
2. Description of the Related Art
For example, Japanese Unexamined Patent Application Publication No. 2006-107663 discloses a hologram recording/playback system that records data by using interference fringes between signal light and reference light and that plays back the data, recorded by the interference fringes, by shining the reference light. As the hologram recording/playback system, the so-called “coaxial system” in which the signal light and the reference light are coaxially arranged to perform recording is available.
FIGS. 10 and 11 illustrate schemes for hologram recording/playback based on the coaxial system, FIG. 10 illustrating a scheme for recording and FIG. 11 illustrating a scheme for playback.
Referring first to FIG. 10, during recording, a light intensity modulator 101 performs, as spatial light modulation, light intensity modulation on incident light from a light source to generate signal light and reference light that are coaxially arranged as shown. The light intensity modulator 101 is implemented by, for example, a liquid crystal panel.
In this case, the signal light is generated by performing spatial light modulation corresponding to record data. The reference light is generated by performing spatial light modulation using a predetermined pattern.
The signal light and reference light generated by the light intensity modulator 101, as described above, are subjected to spatial phase modulation performed by a phase mask 102. As shown, the phase mask 102 applies a random phase pattern to the signal light and applies a predetermined phase pattern to the reference light.
The phase mask 102 performs phase modulation for each pixel unit. The term “pixels” as used herein refer to individual pixels that constitute a modulation surface for light modulation, the modulation surface being included in the light intensity modulator 101. For example, when the light intensity modulator 101 has a liquid crystal panel, one of the pixels that constitute the liquid crystal panel corresponds to one pixel unit mentioned above.
The reason why random phase patterns are applied to the signal light and the reference light is to improve the efficiency of interference between the signal light and the reference light, to reduce DC (direct current) components through diffusion of the spectra of the signal light, and to increase the recording density.
As a result of the light intensity modulation performed by the light intensity modulator 101, light having light intensities modulated into 0 and 1 in accordance the record data is generated as the signal light. The signal light is subjected to phase modulation with a phase of 0 or π, so that light having −1, 0, and 1 (+1) representing amplitudes at a wave surface of the light is generated. That is, when a pixel modulated with a light intensity of 1 is subjected to modulation with a phase of 0, the amplitude is 1, and when a pixel modulated with a light intensity of 0 is subjected to modulation with a phase of π, the amplitude is −1. The phase of a pixel with a light intensity of 0 remains to be 0 with respect to either of a phase of 0 and a phase of π.
FIGS. 12A and 12B show a difference in the signal light and the reference light between a case (FIG. 12A) in which the phase mask 102 is absent and a case (FIG. 12B) in which the phase mask 102 is present. In FIGS. 12A and 12B, large/small relationships in light amplitudes are expressed by color densities. More specifically, in FIG. 12A, black and white represent amplitudes of 0 and 1, respectively, and in FIG. 12B, black, gray, and white represent amplitudes of −1, 0, and 1 (+1), respectively.
The intensity of the signal light in this case is modulated according to record data. Thus, light intensities (amplitudes) of 0 and 1 are not necessarily randomly arranged, thereby promoting generation of DC components.
The phase pattern applied by the phase mask 102 is a random pattern. Thus, pixels whose light intensities of the signal light output from the light intensity modulator 101 are 1 can be randomly divided so that the number of pixels with an amplitude of 1 and the number of pixels with an amplitude of −1 are equal to each other. As a result of such random division into the pixels with an amplitude of 1 and the pixels with an amplitude of −1, it is possible to uniformly scatter spectra in a Fourier plane (a frequency plane, which may in this case be regarded as an image on the medium), thereby making it possible to suppress DC components in the signal light.
Such suppression of DC components in the signal light makes it possible to improve the data recording density.
DC components in the signal light may cause the intensities of the shined light to be concentrated in a recording material. This causes the recording material to react greatly, thus making it very difficult to perform, for example, multiplexed recording. That is, such a phenomenon makes it very difficult to perform multiplexed recording of data to beyond a portion in which the DC components are recorded. Accordingly, suppressing the DC components using the above-described random phase pattern enables data multiplexed recording, thus making is possible to perform high-density recording.
A description will now be given with reference back to FIG. 10.
Both of the signal light and reference light subjected to the phase modulation by the phase mask 102 are condensed by an objective lens 103 and the resulting light is shined on a hologram recording medium HM. Consequently interference fringes (a grating, i.e., a hologram) corresponding to the signal light (a record image) are formed on the hologram recording medium HM. Through the formation of the interference fringes, data is recorded.
Subsequently, during playback, as shown in FIG. 11A, the light intensity modulator 101 performs spatial light modulation (intensity modulation) on incident light to generate reference light. The generated reference light is subjected to spatial light phase modulation by the phase mask 102 so as to be given the same predetermined phase pattern as the phase pattern applied during the recording.
In FIG. 11A, the reference light subjected to the phase modulation by the phase mask 102 is shined on the hologram recording medium HM through the objective lens 103.
In this case, the reference light has the same phase pattern as that applied during the recording. As a result of shining of the reference light on the hologram recording medium HM, diffracted light corresponding to a recorded hologram image is obtained and is output as reflection light from the hologram recording medium HM, as shown in FIG. 11B. Thus, a playback image (playback light) corresponding to the recorded data is obtained.
The thus-obtained resulting playback light is received by an image sensor 104, such as a CCD (charge coupled device) sensor or a CMOS (complementary metal oxide semiconductor) sensor, and data is played back on the basis of signals of the light received by the image sensor 104.